Method of operating blast furnaces



July 1, 1952 B. 5. OLD 2,602,027

METHOD OF OPERATING BLAST FURNACES Filed May 6, 1947 5 Sheets-Sheet l mF} m l a I WET WASHER FIG.

CATO H E R INVENTOR. BRUCE SCOTT OLD y 1952 B. 5. OLD 2,602,027

METHOD OF OPERATING BLAST FURNACES Filed May 6, 1947 a Sheets-Sheet 22.70 a x O 2.60 Q v 9 2.50 1 cc MEAN STACK VELOCITY OF FURNACE GASES,F'E/SEC.

FIG. 2

INVENTOR.

BRUCE SCOTT OLD B. 5. OLD

METHOD OF OPERATING BLAST FURNACES July 1, 1952 3 Sheets-Sheet 5 FiledMay 6, 1947 O O 4 l. 2 2

.rmzo MD E MEAN VELOClTY,FT./SEC., OF GASES IN FREE SPACE ABOVESTOGKLINE INVENTOR.

BRUCE SCOTT OLD BY Patented July 1, 1952 METHOD OF OPERATING BLASTFURNACES Bruce Scott Old, Concord, Mass., assignor, by

mesne assignments,

to Republic Steel Corporation, Cleveland, Ohio, a corporation of NewJersey Application May 6, 1947, Serial No. 746,220

4 Claims.

This invention relates to the operation of blast furnaces under internalpressures substantiall greater than those normally employed.

In his U. S. Patent 2,131,031, patented September 2'7, 1938, Julian M.Avery has set forth the principles of blast furnace operation atincreased internal pressures, and has described the manher of doingsoand the advantages attained thereby. The procedure set forth in thatpatent includes, among other things, supplying'the blast feed gas at asufficiently high superatmospheric pressure, and throttling the gasdischarge from the furnace, to create the desired pressure within thefurnace; This desired pressure may range anywhere from about one toabout ten atmospheres gauge, with advantageous operating results.

The procedure of the said Avery patent has already been adopted in someexisting blast furnaces, with excellent results. Those furnaces, aswould be expected, were designed and built for the normal averageinternal operating pressures of A to 1 atmosphere gauge, and pressureswithin the range of the Avery patent are attained by increasing theblowing pressure and installing throttling means upon the exit gasesfrom the furnace. Present practical considerations require thatoperations under the Avery patent, with presently available equipment,be carried out Within the lower part of his pressure range. This isaccomplished by using blowers capable of delivering the blast at say 30to 40 p. s. i. g. (pounds per square inch, gauge) and imposing a toppressure of up to say p. s. i. g. upon the exit gases. The resultingaverage internal pressures in the furnace are consequently between about1-plus and 2-plus atmospheres gauge. While many existing furnaces cansafely withstand somewhat higher internal pressures, it is at presenteither impossible, or exorbitantly expensive, to obtain blowers whichwill provide adequate wind at pressures in excess of about 40 p. s. i.g. Y

Although higher-pressure furnaces with greater blowing capacity may wellbe built in the future, practical considerations limit the procedure ofthe Avery patent for the present to the existing types of furnace,wherein suitable modifications in blowing capacity and in the throttlingand handling-of the exit gases are employed for increasing furnacepressure. Nevertheless, it would be highly advantageous to attain inexisting furnaces the increased benefits which would flow fromthe-operation of the Avery process understill hi her pressures and ratesof blowing.

It is now found, in accordance with the present invention, that suchbenefits may be-attained, by proper control of operating conditions,while still operating the furnaces at the above-stated pressures of 30to 40 p. s. i. g. blast and up to 20 p. s. i. g. top pressure, orthereabout. Also, and within the scope of the present invention, it ispossible to improve production, operations, and economies over those setforth in the Avery procedure, when operating at higher pressures-withinhis range (e. g. up to 7 atmospheres gauge average static pressure), aswell as at the lower pressures (e. g. 2 atmospheres gauge, averagestatic pressure) at which the present type of furnaces can be operated.

The procedure of the present invention also makes possible theutilization of the finer particle size ores which increasinglyconstitute the major reserves because of the depletion of highergradeores. And furthermore, even when using such finer ores, the procedure ofthis invention assures that the amount of solids in the exit gases (i.e. flue dust) is maintained well below that occurringwith normaloperation.

These several advantages are brought about, in accordance with thepresent invention, by the proper control of certain operating variableswithin specified limits, as set forth below. These operating variablesare, in particular, the velocity of the gases through the furnace, andthe burden ratio. a

The principal economies which can be realized in the operation of ablast furnace with the present invention are those incident toincreasing iron production; decreasing coke rate; decreasing flue dustproduction; and obtaining more uniform iron quality with a lesser percent of casts falling outside specification limits. Proper control ofthe mean linear stack velocity of thereducing gases within theinterstices of the furnace charge makes possible a substantial increaseabove normal in the amount of wind blown 'pcr unit of time and hence inthe iron production of the furnace; .it also increases the thermalefficiency of the furnace by increasing the efficiency of heat transferand decreasing the heat losses per ton of -iron produced. In addition, Ihave found that this control'of gas velocity makes it possible toincrease substantially the burden ratio with resulting fuel economy; tocarry higher blast temperatures which .further effectscoke saving; todecrease thepressure dropthrough. the: furnace stock column to bringabout smoother operation with lessslipping and hanging; and to minimizemarkedly "the dust blown out of the furnace; For

the greatest overall economy this control can best be exercised throughenriching the air blast with oxygen while maintaining the highestaverage pressures that the shaft of the furnace is capable ofwithstanding, by throttling the exit gases. In accordance with thepreferred embodiments of my invention, I now describe in what followsthe operation of a modern type blast furnace. The normal amount of windblown per minute (or normal blast rate) referred to above is the normalblast rate at which a given furnace is designed to operate using toppressures of not over about 2.5 p. s. i. g. Methods of calculating suchblast rates are well known and are used by all steel companies.

This invention will now be described in detail with reference to theaccompanying drawings, wherein:

Fig. 1 represents, largely diagrammatically, and partly in section, aside elevation of a blast furnace and associated equipment adapted tocarry out the procedure of this invention;

-Fig. 2 is a chart showing mean stack velocity of furnace gases plottedagainst burden ration; and

Fig. 3 is a chart showing velocity of furnace gases in the free space atthe top of the furnace just above the stock line plotted against fluedust produced per ton of iron.

Fig. 1 illustrates a blast furnace Iii and auxiliary equipment, havingcertain special features found to be desirable for operation at highaverage static pressure, but which are otherwise of more or lessconventional design.

In operating the blast furnace according to the new method and with theequipment described, turbo blower l2 operates to supply the blast, andthe average static pressure in the shaft of the furnace I is raised tothe desired level by throttling the discharge of gases, as by means ofthe throttling valve 49. The blast pressure is increased an appropriateamount which is dependent on the pressure at the top of the furnace andthe pressure drop through the stock column. which is dependent onseveral factors such as the velocity of the gases through the stockcolumn, average static pressure in the shaft, and character of thecharge. The wind blown, defined as cubic feet of blast per minutedelivered to the furnace. is then regulated at the proper level by meansof turbo blower speed control and any enrichment oxygen which is fedinto the conduit [4 beyond the turbo blower. The blast is compressed byturbo blower l2 and passes through conduit [4, stove l5, and bustle pipe16 to tuyres 18. Enrichment oxygen may be supplied to the blast in anydesired manner, e. g. by the use of a separatorffl which may for examplebe a Linda-Frankl type separator. Byproduct nitrogen from the separatorpasses off at 22, while the oxygen is supplied to the blast ata-suitable place as determined by the relative pressures of the exitoxygen and of the blast. For example the oxygen may be passed viaconduit 24 controlled by valve 26 to conduit 28 which supplies air tothe turbo blower, or it may be passed via conduit 30 (controlled byvalve 32) to conduit l4 after the turbo blower. Alternatively to thelatter procedure, compressor may be used to raise the pressure of theoxygen supply to conduit M, the oxygen passing to the latter conduitthrough conduit 36 controlled by valve 38.

As already indicated, a static pressure is imposed upon the furnaceshaft by any suitable means, for example a'pressure operated throttlingvalve 40, which as shown consists of three pipes with a butterfly valvein each, and which discharges into the Cottrell precipitator. To sealthe top of the furnace l0 against leaks because of the pressure withinthe furnace, hopper 42 and lower bell 45 may each be made in one piece,with their contacting surfaces 48 and 41, respectively, beinghard-surfaced. Alternatively, a flexible sealing member such as aninternallywater-cooled hose may be associated with the lower surface 46of hopper 42, to ensure a gastight fit against bell 44.

Hard surfaced bleeder valves 48, 49 are provided in the customaryposition, at the top of uptakes :i8a and 49a, respectively. Gas from thefurnace passes through downcomers 50, 50a, thence into and through drydust catcher 5|, and thence through conduit 52 to wet washer 53. Thelatter has a control system and overflow 54, 56, to maintain the waterseal in the wet washer within predetermined limits in spite ofvariations in pressure due for example to slipping or hanging of thecharge. 7

The gas discharged from washer 53 passes through conduit 55 tothrottling valve 40 already referred to. Conduit 55 extends ondownwardly to form a blind end for collecting any water and solidscoming over with the gas; these collect at the bottom and may be removedfrom time to time through a suitable gate '51.

Instead of using a throttling valve M for controlling the gas pressurewithin the furnace, this gas pressure may be maintained by a powerrecovery system of the type set forth in the aforementioned Avery Patent2,131,031 and in Avery Patent 2,192,885. In that event, 53 may be 'a drydust separator auxiliary to dust catcher 5|. and will be a valvelessconduit leading to the power recovery mechanism, which latter thenserves as the throttling means to maintain the pressure in the furnace.

The wet washer (or dust separator) 53 isalso provided with relief outlet58 communicating with another bleeder valve 59, the latter being locatedadjacent the bleeder valves 48 and 49 at the top of the furnace so thatthe gas discharge may be made at a safely high level away from theworking space at the bottom of the furnace. The outlet line -58 andbleeder valvel 59 serve to bleed clean gas from the washer 53 duringminor slipping of the furnace. H V I V Equalizer line 60 controlled byvalve 62 permits the flow of washed gas fromoutlet 58 of wet washer 53into the space between the 'bells, 'to equalize the pressure above andbelow lowerfbell M sufficiently to permit dumping the latter. Reliefline 64 controlled by valve 65 provides a means for exhausting the spacebetween the bells. so that the upper bell '68 may be dumped.

Among the structural features described above are a number which are ofvalue in operating the furnace in accordance with the present invention.These are in particular the hard-surfaced contacts, or the flexiblesealing members. 'forbell s4 and hopper 42, the wet washer control andoverflow system 54, 56, and the system for bleeding clean gas from Wetwasher'53 through outlet 58 to bleeder valve 59 and line 60. Thesefeatures are not, however, claimed herein, but form the subject mattersof other applications for patent as follows: Slater, Serial No. 775,016,filed September 19, 1947, now abandoned; Latham, Serial No. 763,821,filed July 26, 1947, now Patent No. 2,599,334; J anecek and Bahney,Serial No. 773,680, filed September 12,1947, now-Patent N0.

2,602,027 6 2,585,779; and Le Viseur and Larson, SerialNo'. at least2.45 and even to 2.7 or 2.8, by throttling 771,870, filed September 3,1947, now Patent No. the exit gases to decrease the mean stack gas2,585,800, relating to hard-surfaced contacts, velocity to between about45 and 55 ft./sec., which flexible sealing members, wet washer system,and latter figure is just below the aforementioned system for bleeding,respectively. 5 upper critical stack gas velocity. The saving of By wayof example, assume that operation by over 200 lbs. of dry coke per tonof iron thus the new method herein set forth is planned for madepossible by controlling mean stack gas a blast furnace and burden of thefollowing charvelocity is of cardinal importance in view of theacteristics (which are commonplace): dwindling supply of metallurgicalcoke in this country.

g zi g fi figf" cubic ft By normal blast rate is meant the blast de- Wrking hei ht k livered to the furnace when operating at normal Stack ift top pressures and blowing with ordinary air (2l% Ferrous burden (56%Mesabi ore, 36% Old Range ii fi at liatestas as practlcible Whlch ore 8%Open Hearth slag) 50% F l isusualyse by he endency of he urnace 5%carbon to hang and produce a prohibitive amount of dust. This normalblast rate is su-flicient to pro- Calculations can then be made of themean duce iron at a tonnage approximately equal to gas velocity ofthereducing gases through the that of the rating of the furnace.

working volume of the furnace (also referred to By burden ratio is meantthe ratio of pounds hereinafter and in the claims as mean stack gas offerrous material charged, such as ore, scale,

velocity) by the following method: open hearth slag, etc., to the poundsof dry coke Mean stack gas velocity in feet per second: charged, perunit of time such as per round or (wind delivered in cu. ft./sec. 60 F.,14.7 per day. This ratio is commonlyus'ed in the steel p. s. i. a.) X(absolute pressure and temperature industry. In cases where the chargecontains corrections for the average of the conditions in appreciableamounts of carbonate ores, such as the stack) (gas expansion factor)(average siderite, the weight of combined CO2 in the eifective crosssectional areaof furnace'working ferrous material charged is to beexcluded from volume). V the figures for pounds of ferrous materialSpecifically, as applied to a furnace having the charged, in calculatingthe burden ratio. dimensions and burden given above, blown with Theincreased burden ratios attained in the air at 75,000 C. F. M. wind,with a blowing prespractice of the present invention may be exsure of 21p. s. i. g. and a top pressure of 2.5 pressed in burden ratio figures,as already indip. s. i. g., and having a tuyere temperature of catedherein, or they may be expressed as per cent 2800 F. and a toptemperature of 300 F., all of increase over the burden ratios customaryin prior which are normal conditions for such a furnace, practice, i. e.without pressure operation. A'fair the mean stack gas velocity iscalculated as folvalue, somewhat on the high side, for the burden lows(note: p. s. i. a. pound-s per square inch, ratios attained in suchprior practice is, as absolut already indicated, 2.3. Hence the burdenratio 75000 7 20l0 l 3 5 so 26.4 520 54 Mean stack gas velocity:

Where 26.4=average absolute static furnace pressure I blast pressure+toppressure =67.2 ft./sec.

=calculated wind delivered, cu. ft./sec. 60 F.,'l4.7 p. s. i. a; r.

2010 average absolute furnace temperature (R.) (W) 460 460 520 =abso1uteair temperature (R.) 60 +460 rm n 1,35-gas expansion factor in topgas-5&5

54=average effective cross-sectional area of furnace working volumeworking vo1ume 0. 10 X 44,346 V working height 82 It has been found, inaccordance with the pres- 60 value of 2.45 attainable under the presentinvenent invention, that the mean stack gas velocity tion represents anincrease of 6.5% over the said so calculated controls the burden ratiothe furnormal value, while the still higher burden ratios nace cancarry, 1. e., within limits, the lower this of 2.7 or 2.8 herebyattainable represent increases velocity, the higher the permissibleburden ratio. of about 17% and about 22% respectively. Furthermore, whenthe calculated mean stack gas The solid line of Fig. 2 represents thecalcu- =per cent voidsX velocity is plotted against burden ratio as inlated values for burden ratio vs. mean stack gas Fig. 2, an uppercritical mean stack velocity of velocity, which values are also theaverage values gases in the furnace stock column of about 60 found inactual operation. Obviously, the conft./sec. is found below which theburden ratio'can trols, and conditions of operation, and constituberaised sharply, to a point where it levels off on 0 cuts of the charge,of any blast furnace can never reaching a limiting value as dictated bythe therbe maintained with such exactness that the re-v mal requirementsof the smelting process. Thus sults Will always fall upon the solid lineof Fig. 2. the burden ratio of 2.3 or less carried by the fur- They do,however, fall within a fairly restricted nace at the normal mean stackgas velocity of range indicated by the dashed lines on either side about65 ft,/se c. -can ,be increased markedly, to 7? of the solid line ofFig; 2.

2,602,027 7 8 The main reasons found, in carrying out the produced inthe nearl vertical portion of the present invention, for thesaving incoke through curve of Fig. 3, as the mean stack gas velocity control ofmean stack gas velocity in the ranges controls to a large extent theslipping and chanof pressure operation allowableby limitations in nelingtendencies of :the charge by controlling the present turbo blower andblast furnace design pressure drop .through the stack. Thus the are: (1)More effic'ient use of the reducing power higher the mean stack gasvelocity the rougher of the furnace gases'through better gas'distributhe operation and the greater is the amount of tion in thestock column and-longer time of condust thrown into the space above thestock line tact, both of which are effected by lower gas where it can'becarried over into the downcomers velocity. The justification for thisstatement is 10 by free-space gas velocities in excess of the critifoundin the decrease occurring in (IO/CO2 ratio cal value of about 4.5ft./sec.

of D gases leaving the furnace as the velocity Although, as will benoted from the foregoing decreases; (2) More efficient heat transfersample calculations, there isno direct connection through more uniformgas fiow and longer time between the mean stack gas velocity and thefreeof contact of the'hot ascending gases'with the space gas velocity,the point of critical velocity of descending charge, which also resultsfrom deabout 4.5 ft./sec. of the latter corresponds roughly creasedvelocity. Thus the temperature of the to a velocity of about 45 ft./sec.for the former. exit gases from the top of the furnace has been Thereare three methods "of decreasing the found to be a direct function ofthe velocity for mean stack velocity of gases through the furnace agiven average static furnace gas pressure; and charge:

(3) Utilization of higher blast temperatures at ('1) Increasingtheaverage s'ta'tic pressure withthe higher pressures in the shaft of thefurnace. in the furnace Control of-gas velocity permits another impor-(2) Decreasing the wind volume blown tant saving to be effected in theblast furnace -03) Enriching the air blast with oxygen.

process. The fine dust produced per ton of iron It is obvious thatmethod (2) is undesirable, has been found to be a function of thefree-space except possibly during times of business depresgas velocity,as shown in Fig. 3. This free-space sion, as low iron tonnagesareproduced-when low gas velocity is that of the gases in the free spacewind volumes are utilized.

just above the stock at the stockline of the fur- In Table I, below, areshown data and calculanace. It is calculated by the following method: 39tions for operations of a furnace having the par- Free-space gasvelocity just above the stock at ticular dimensions already given, atnormal presthe s'tockline, in feet per second: (wind delivered sure of2.5 p. s. i. g., and at increased top presin cu. ft./sec. 60 R, 1417p.s. i. a.)' (absolute sures of 10 p. s. i. g. and 20 p. s. i. g., in eachcase pressure and temperature corrections for condiwith ordinary air,with 25% oxygen air, and tions at the stockline) (gas expansion factor)with oxygen-air. 'By 25% oxygen air and (cross-sectional area offurnac'e'at the stockline'). 40% oxygen air are meant'respectively aircon- Specifically, as applied to afurnace having thetaining'substantially 25% O2 and 74% N2, and dimensions and burdenalready specified above, air containing substantially 4=0% O2 and 59%blown with air at 75,000 0.1. M. wind, with a N2, all figures being byvolume. In the column top pressure of 10 p. s. i. g. and a top tempera-4.0 headed Wind (or 02) Delivered (C. F. M.) are ture of 300 F., thesaid free-space gas velocity is figures in parentheses showing the C. F.M. of calculated as follows: oxygen in the wind delivered, e. g. 90,000C. F. M.

Free-space gas velocity= X =4.3 ft.'peI'-Sec.

Where 75000 o =calculated wind delivered, cu. ft./sec. 60 F., 14.7p.s. 1. a.

14.7 g1. 7 -abso1ute pressure correction for top pressure 10 p..s. 1. g.

460+300 7 60 o 460 60 -absolute temperature correction for toptemperature @300 F.

l.35=gas expansion factor= Again it will be noted that a criticalvelocity is of 25% oxygen air contains 22,500 C. F. M. oxygen. reached,this being one below which the dust 60 Normal air isfigured at 21%oxygen. The showproduction of the furnace becomes-inconscqueningsofFigs. 2 and 3, and the figures under N0 tial. Thus if one controlsthefree-space velocity Air *Enrichmen in Table I, are based upon exatbelow about-4.5 ft./sec. the flue dust produced tensive series of actualoperatingrun while the will be of the order of 90 lbs. or less per tonof figures for the balance of Table I are obtained iron, instead of anormal value of over 200 by calculations basedupon the resultspractically lbs/ton of iron for operation at 75.000 C. F. 'M. attainedinsuchoperatingruns, upon experience delivered wind at 2.5 p. s. i. g.top pressure. The gained therefrom, and upon well-founded theoresultingsaving in iron yield really raises the retical considerations.

effective burden ratio charged. In addition, As an example, withreference toTable I, assavings will result from having to sinter onlythe sume that it is desired to supply to this furnace smaller amount offlue dust for'rechargin into about 22,000 C. F. M. of oxyge in th wind,This the blast furnace. The mean stack gas velocity, is not practicallyfeasible with ordinary air Withif so high that the free-spaceigasvelocity is above out added top pressure, because among other thecritical dust carry-over value found, is imthings, the gas velocitywould be sogreat as to portant in determining how much dust will be blowmuch of the charge out of "the furnace.

Even with added top pressure, it is hardly practicable with presentlyavailable equipment, since the blowing capacity and pressures requiredare in general too high. However, by enriching the blast with oxygen,and imposing practicable pres sure and blowing conditions, 22,000 C. F.M, of oxygen in the wind can be readily provided. Thus, by blowing90,000 0. F. M. of 2 oxygen air at p. s. i. g. top pressure, goodoperating results in accordance with the present invention areattainable. Similarly good results are attainable by blowing 60,000 C.F. M. of about 37% oxygen air, at about 10 p. s. i. g. top pressure.

TABLE I the increase in burdenfratio possible through gas velocitycontrol. should I prove highly advantageous. Since the ore reservesoftheUnited States are now largely made up of. lean ores, the importanceof gasvelocity control cannot be overemphasized. i

As is clear from this table and from the foregoing disclosure, as wellas from Fig. 2, the desirable results of the present invention areattained by imposing a top pressure onthe exit gases, and blowingsufiicientair to efiect a mean stack velocity of the furnace gases ofless than ft. per sec. and preferably not over 55 ft.

Burden ratio, iron production, dry coke rate and dry flue dustproduction for varying furnace gas velocities and degrees of airenrichment O W121i TOP P- Blast $233 02; Burden 3.34 553 iai 535 1?"Delivered sure Pressure Velocity R500 n51 Tons 1.55.0011 LbsJton (0.1.m.) (p. s. i. g.) (p. s. l. g.) (ft/Sec) Day Iron Iron NO AIR ENRICHMENT2. 5 31. 3 79.1 1 105,000 10 38.8 53. 9 2. 315 1, 500 1, 530 255 22,05020 4s. 3 50. s 2. 525 1, 935 1, 430 e 25% OXYGEN .4111 (BY VOLUME) 2. 521 70. 2 2. 26 1, 404 1, 720 310 75,000 10 2s. 5 54. 7 2. 515 1, 575 1,544 13,750 20 3s. 5 42, 3 2. 75 1, 730 1, 412 34 2. 5 20. 3 7G. 4 2. 2251, 650 1, 750 400 90,000 10 33. 8 G0. 6 2. 365 1, 765 1, 643 195 22,50020 43. 3 47. 5 2. 595 2, 030 1, 440 57 40% OXYGEN AIR (BY VOLUME) 2. 521 80. 9 2. 215 2, 200 1. 754 400 75,000 10 2s. 5 53. 0 2. 325 2, 320 1,570 150 30,000 20 3s. 5 4s. 7 2. 5s 2. 700 1, 450 50 X Probablyinoperable.

The data and calculations given in Table I are based upon the results ofoperatin a blast furnace of the size hereinbefore described on socalledstraight ore burden using untreated Lake ores, as previously outlined,or on containing amounts of sinter, scale or scrap such that the ironcontent of the ferrous materials in the burden is approximately 48-54%,and for coke containing about 86.5% C and about 3 to 8% moisture. Fordifierent types of burdens the values in Table I would have to bealtered by methods familiar to one skilled in the art. Thus for higheriron content burdens, for exampl those having higher than 60% Fe in theferrous materials charged, the normal burden ratios would be so highthat relatively little increase would result from gas velocity control.For lower iron content burdens, for example ,25% Fe or lower,

per sec., while maintaining an average 1555 15 pressure within thefurnace of- 50155555115 atmosphere gauge. The top pressure so imposedshould be at least 5 p. s. i. g., and for most furnaces will, forpractical purposes, be in the general order of 10 to 20 p. s. 'i'. g.'For furnaces operatmgunder say seven atmospheres out in Table I is itsflexibility. I erator can adjust the gas velocity, and hence shown inthe foregoingdisclosure and in Table I. It will be noted, especiallyfrom Table I, that the high iron production and the low'coke rate andlow flue dust are attained with burden ratios in excess of 2.45, andmean stack gas velocities of not more than about 55 ft./sec. Preferablythis stack velocity is maintained above about 35 ft./sec., althoughlower velocities may be used with some decrease in coke rate from whatcould be attained at a higher velocity (but below about 55 ft./sec.),but this will generally be ofiset by increase in blowing costs ordecrease in productivity. In general, the best results from theoperation of the present invention are attained with a burden ratio ofabout 2.7 to 2.8 (i. e.

about 17% to 22% above normal), a mean stack gas velocity of 35 to 45ft./sec., and a free-space gas velocity of less than 4.5 ft./sec. Suchoperations are in the zone of the upper inflexion point of the curve inFig. 2, and below the point in Fig. 3 at which the flue dust productionmounts nearly vertically.

As is evident from Fig. 3, there is no critical lower limit forfree-space gas velocity. However, other conditions being equal, as meanstack gas velocity decreases, so does free-space gas velocity, althoughnot necessarily in the same ratio. At the lower ranges of mean stack gasvelocity herein set forth, i. e. about 35 ft./sec., the free-space gasvelocities will generally be in the order of 3 to 4 ft./sec.

From Table I it will be noted that, normal top pressure of about 2.5 p.s. i. g., nor-- mal blast pressure-of about 21 p. s. i. g., normalmaximum wind volume delivered to the furnace of about 75,000 C. F. M.,and normal gas velocity of about 67 ft./sec., the burden ratio is about2.28, the iron tonnage is about 1187, the coke rate about 1710, and theflue dust about 260 lbs. per ton of iron. Ordinarily, as one increasesthe blast rate, the coke rate and flue dust production also increases.However, it will be seen that one can, through gas velocity control,increase the unenriched blast rate to 105,000 C. F. M. and yet decreasethe coke rate from the normal value of 1710 to 1480, and decrease theflue dust by over 100 lbs. per ton of iron, and increase production ofiron by around 700 tons per day. Furthermore, with 25% oxygen air at theotherwise normal operation of 15,000 C. F. M. u

blast rate at 2.5 p. s. i. g. top pressure it will be noted that thecoke rate will actually increase unless gas velocity control is utilizedto increase the burden ratio. When this is done a saving of about 300lbs. of coke per ton of iron is made possible, as well as a substantialsaving in flue dust. The importance of these savings will be recognizedwhen it is realized they represent a manufacturing cost saving of over$1.00 per ton of iron.

Another feature of gas velocity control brought Thus the opburden ratio,to suit his operating conditions (turbo blower'and furnace top pressurelimita tions, and prevailing economic situation and plant costs) so asto balance coke rate, iron production, flue dust production, and blowingcosts in order to operate the blast furnace at the optimum point ofeconomic balance.

By using the new variable controls of this invention, gas velocity andblast enrichment, the operator can achieve economies of different kindsand adapt the furnace operation to the conditions and demands of theoverall steel plant, of

for the 12 which the blast furnace is usually an integral unit. Forexample, if the coke supply is limited and the demand for pig iron abovethat available from normal operation of the furnace, the operator canadapt the burden ratio (which controls coke consumption) to about 2.7 bydecreasing gas velocities through the furnace to say 40 ft. per see. bygoing to a top pressure of 20 p. s. i. g. Under these conditionsproduction would be increased about 280 tons per day with substantiallythe same total daily coke consumption; or if the furnace could not beoperated for purely physical reasons at such internal pressures, thenthe operator could achieve the same results by going to about 10 lbs.top pressure and enriching the blast in oxygen. With this same toppressure limit and the demands for still greater production the operatorcould increase the blast with some sacrifice in coke economy or maintainthe same desirable high burden ratio through blast enrichment, whilecontinuing to operate under the maximum top pressure which can be safelyused in the furnace. From Table I it is therefore evident that widelyvarying conditions of operations can be realized with any given furnacethrough the manipulation of the variables of gas velocity and blastenrichment while operating under higher than normal top pressures. Theflexibility achieved by the procedure of the present invention istherefore of the greatest practical importance. In fact, with itsprovision for proper control of gas velocity in the furnace, thisinventien opens up an entirely new order of magnitude of production tobe attained from present day blast furnaces, at efficiencies nothitherto believed to be possible.

I claim:

1. The method of producing iron from iron ore in a blast furnaceutilizing a burden ratio which is high for the grades of coke and ore,with consequent decrease in coke consumption, comprising blowing aircontaining approximately 25% to 40% by volume of oxygen at a high blastrate sufficient to produce a mean stack velocity through the stockexceeding 55 linear feet per second at a top pressure of about 2.5pounds per square inch gauge, throttling the outlet of the furnace toimpose suficient top pressure on the exit gases to limit the mean stackvelocity to the range of 35 to 55 linear feet per second through thestock while holding the blast at said high rate, said top pressure beingadjusted in the range of 5 to 20 pounds per square inch gauge to imposean average internal static pressure in the furnace of greater than 1atmosphere gauge.

2. The method as defined in claim 1 wherein the mean stack velocity ofthe gas through the furnace is limited to the range of 35-45 linear feetper second.

3. The method of producing iron from iron ore in a blast furnaceutilizing a burden ratio which is high for the grades of coke and ore,with consequent decrease in coke consumption, comprising blowing aircontaining approximately 25% to 40% by volume of oxygen at a high blastrate sufiicient to produce a mean stack velocity through the stockexceeding 55 linear feet per second at a top pressure of about 2.5pounds per square inch gauge, throttling the outlet of the furnace toimpose sufficient top pressure on the exit gases both to limit the meanstack velocity to the range of 35 to 55 linear feet per second throughthe stock and to limit the linear gas velocity above the stock to lessthan 4.5 feet per second, while holding the blast at said high rate,said top pressure being adjusted in the range of about to pounds persquare inch gauge to impose an average internal static pressure in thefurnace of greater than 1 atmosphere gauge.

4. The method of producing iron from iron ore in a blast furnaceutilizing a burden ratio which is high for the grades of coke and ore,with consequent decrease in coke consumption, comprising blowing gascontaining at least approximately by volume of oxygen at a blast ratesufiicient to produce a mean stack velocity through the stocksubstantially exceeding 55 linear feet per second at a top pressure ofabout 2.5 pounds per square inch gauge, throttling the outlet of thefurnace to impose suflicient top pressure on the exit gases to limit themean stack velocity to the range of to linear feet per second throughthe stock, said top pressure being adjusted to above about 5 pounds persquare inch gauge to impose an average internal static pressure in thefurnace of greater than 1 atmosphere gauge.

BRUCE SCO'I'I' OLD.

14 REFERENCES CITED The following references are of record in the fileof this patent:

UNITED sTATEs PATENTS Blast Furnace and Steel Plant, June 1945,

pages 699 to 707.

Iron and Steel Engineer, February 1946, pages 88 to 94, and 101.

Transactions, American Institute of Mining and Metallurgical Engineers,vol. 131, pages 108, 109, and 110.

1. THE METHOD OF PRODUCING IRON FROM IRON ORE IN A BLAST FURNACEUTILIZING A BURDEN RATIO WHICH IS HIGH FOR THE GRADES OF COKE AND ORE,WITH CONSEQUENT DECREASE IN COKE CONSUMPTION, COMPRISING BLOWING AIRCONTAINING APPROXIMATELY 25% TO 40% BY VOLUME OF OXYGEN AT A HIGH BLASTRATE SUFFICIENT TO PRODUCE A MEAN STACK VELOCITY THROUGH THE STOCKEXCEEDING 55 LINEAR FEET PER SECOND AT A TOP PRESSURE OF ABOUT 2.5POUNDS PER SQUARE INCH GAUGE, THROTTLING THE OUTLET OF THE FURNACE TOIMPOSE SUFFICIENT TOP PRESSURE ON THE EXIT GASES TO LIMIT THE MEAN STACKVELOCITY TO THE RANGE OF 35 TO 55 LINEAR FEET PER SECOND THROUGH THESTOCK WHILE HOLDING THE BLAST AT SAID HIGH RATE, SAID TOP PRESSURE BEINGADJUSTED IN THE RANGE OF 5 TO 20 POUNDS PER SQUARE INCH GAUGE TO IMPOSEAN AVERAGE INTERNAL STATIC PRESSURE IN THE FURNACE OF GREATER THAN 1ATMOSPHERE GAUGE.